Long-term complications in normal tissue lying in the treated volume can severely limit the usefulness of radiation therapy. In radiation therapy for breast cancer the complex anatomy of the chest wall, breast, and the lymphatic system draining the region necessitates large, complex treatment volumes in close proximity to vital normal tissues in the thorax particularly the heart and lungs, which are at risk for serious late effects. The overriding long term objective of this proposal is to improve methods of breast cancer radiation therapy by better confining the radiation to the tumor target volume for adjuvant and primary treatment in order to minimize long-term complications of incidentally irradiated normal tissues. The improvement we propose, electron arc therapy, rests on the physical characteristics unique to moving electron beams and the capabilities of modern medical linear accelerators which provide a selection of multiple electron beam energies, rotational capabilities, and developing computerization which will add sophisticated control to the operation, record keeping, and safety of conformational therapy. In the initial period of funding for this project a number of technical optimizations were implemented for electron arc adjuvant chest wall irradiation and 64 treated patients were studied demonstrating the efficacy for local control, excellent cosmesis, safety and superior dose-volume relationships in lung and heart as compared to tangential photon treatment. In the renewal period we propose further technical developments for optimization of adjuvant therapy, clinical documentation of the safety of the procedure by searching for evidence of pulmonary and cardiac dysfunction in asymptomatic patients treated by electron arc, and the adaptation of the technique to the treatment of the intact breast through use of a novel range shifter to deliver uniform dose throughout the entire treatment volume.